1. Field of the Invention
The present invention relates to a Liquid-Crystal Display (LCD) device and more particularly, to the structure of a backlight unit of a LCD device.
2. Description of the Related Art
The LCD device has been extensively used as a monitor of the so-called Office Automation (OA) apparatus, audio-visual (AV) apparatus, mobile terminal device, and so on. This is because the LCD device has preferable characters such as compactness, thinness, and low power-consumption.
The LCD device is divided into several types according to the illumination structure, such as the direct illumination type, and the edge light type. The edge-Light type LCD device comprises, for example, a backlight unit having a fluorescent lamp or lamps as the backlight source, a reflector, a light guide plate, a reflection sheet, and so on; an optical sheet for generating uniform illumination light from the backlight; a LCD panel having a pair of opposing transparent substrates and a liquid crystal layer interposed between the substrates; a casing for receiving and fixing these members; and various substrates held by the casing.
As the fluorescent lamp for the backlight unit, generally, a discharge tube containing a gaseous mixture of mercury (Hg), argon (Ar), and neon (Ne) gases is used. With this tube, mercury atoms existing in the tube are ionized by excitation due to discharge to emit ultraviolet (UV) rays. The UV rays thus emitted are irradiated to the fluorescent material coated on the inner wall of the tube, thereby exciting the material. Thus, visible light is emitted toward outside through the wall of the tube, turning the said tube on.
While the fluorescent lamp is discharging, heat is generated at the pair of electrodes (i.e., the anode and the cathode) located in the tube at its each end due to electrical loss and as a result, the lamp has a temperature distribution such that the temperature is the highest at its ends. The heat generated at the electrodes will flow or dissipate to the outside due to thermal conduction by way of the glass that forms the valve of the lamp and the conductors of the cables or wires that are connected to the electrodes.
FIG. 1 schematically shows the connection structure of the fluorescent lamp of a conventional backlight unit for the edge-light type LCD device. As seen from FIG. 1, the conventional backlight unit 106 comprises a fluorescent lamp 110, a flat cable 111, and two sheathed wires 112a and 112b. One end of the flat cable 111 is connected to one end of the lamp 110 by way of a lead 121 of the lamp 110 with a solder 122. The other end of the flat cable 111 is connected to one end of the sheathed wire 112a with a solder 123. The other end of the sheathed wire 112a is connected to a connector 124. One end of the sheathed wire 112b is connected to the other end of the lamp 110. The other end of the sheathed wire 112b is connected to the connector 124. The connector 124 is connected to a start-up circuit 113.
The flat cable 111 has the structure shown in FIG. 2, which comprises a belt- or tape-shaped conductor 114a and an insulator 115a covering entirely the conductor 114a. The insulator 115a is formed by a pair of insulating films located at each side of the conductor 114a. Both side edges of the insulating films are adhered to each other to confine the conductor 114a therein. The conductor 114a may be formed by adhering a patterned conductor foil or by printing a conductor film. The cable 111 may be bent in its thickness direction; however, it is difficult to be bent in its width direction.
The sheathed wires 112a and 112b have the same structure as shown in FIG. 3, which comprises a round conductor 114b and a round sheath or insulator 115b covering entirely the conductor 114b. The conductor 114b is formed by a plurality of thread-shaped cores 114bb. The conductor 114b (i.e., the cores 114bb) is entirely covered with the sheath 115b. The sheath 115b is formed by a tube-shaped insulating resin.
The conductor 114a of the flat cable 111 is smaller in cross-sectional area than the conductor 114b of each of the sheathed wires 112a and 112b. 
The above-described conventional backlight unit 106 is disclosed, for example, in the Japanese Non-examined Patent Publication No. 9-55112 published in 1997.
With the conventional backlight unit 106 shown in FIG. 1, the flat cable 111 and the sheathed wire 112b, which have different shapes or structures as shown in FIGS. 2 and 3, are respectively connected to the ends of the fluorescent lamp 110. Since the cross-sectional area of the conductor 114a of the cable 111 is smaller than that of the conductor 114b of the wire 112b, thermal conductivity difference arises between the cable 111 and the wire 112b. This means that the heat dissipation performance at the electrode of the lamp 110 to which the flat cable 111 is connected is less than that at the electrode of the lamp 110 to which the sheathed wire 112b is connected. As a result, the temperature at the electrode of the lamp 110 to which the flat cable 111 is connected will be higher than that at the electrode of the lamp 110 to which the sheathed wire 112b is connected during operation. In other words, the temperature of the lamp 110 will not be equal at its ends during operation. In summary, the cross-sectional area difference between the conductors 114a and 114b of the cable 111 and the wire 112b induces a difference of their heat dissipation performances, resulting in a temperature difference between the ends of the lamp 110.
Mercury gas confined in the fluorescent lamp 110 has the following property. Specifically, when temperature difference occurs between the ends of the lamp 110, mercury ions induced by discharge in the lamp 110 are likely to move toward the electrode with a relatively low temperature. Therefore, the mercury ions will deviate from their uniform distribution due to the temperature difference between the ends of the lamp 110, resulting in a mercury ion distribution where the density of the mercury ions in the vicinity of the electrode with a relatively low temperature is higher than that in the vicinity of the electrode with a relatively high temperature. Such the deviation of the mercury ion distribution as seen in the fluorescent lamp 110 is termed the “cataphoresis phenomenon”.
Because of the above-described uneven distribution of the mercury ion in the lamp 110, normal discharge is unable to occur in the region of the lamp 110 where the mercury ion density is low (i.e. the mercury ions are thin). Instead, rare gas discharge will occur in the said region using the argon and neon ions confined in the lamp 110 along with the mercury ions. As a result, burnt orange light will be emitted and color unevenness will happen on the display screen of the LCD device after the operation or illumination of several thousands of hours.
For example, when the conductor 114a of the flat cable 111 has a cross-sectional area of 0.08 mm2 and the conductor 114b of the sheathed wire 112b has a total cross-sectional area of 0.2 mm2, a temperature difference Δt between the ends of the lamp 110 will be 32° C. The temperature at the end of the lamp 110 to which the flat cable 111 (the cross-sectional area is relatively small) is connected is higher than that to which the sheathed wire 112b (the cross-sectional area is relatively large) is connected.
With the conventional backlight unit 106 of FIG. 1, as seen from the above explanation, the cable 111 and the wire 112b whose conductors 114a and 114b have different cross-sectional areas are respectively connected to the ends of the lamp 110. Therefore, the heat dissipation performance is not equal at the ends of the lamp 110 and thus, considerably large temperature difference will occur between these ends. Due to the temperature difference, the mercury ions in the lamp 110 are likely to move toward the end of a relatively low temperature to generate a deviation of the mercury ion distribution in the lamp 110. As a result, there is a problem that display quality degradation of the LCD device is caused by the color unevenness on the display screen and that the long-term reliability deterioration of the LCD device is caused by the operating life decrease of the lamp 100.